There has been known, as an autonomic nerve function related to blood pressure fluctuation, a baroreflex function for keeping a value of a blood pressure in a normal range. Deterioration in this baroreflex function is considered to cause hypertension or orthostatic hypotension. Therefore, a technique for measuring the sensitivity of the baroreflex function has been proposed. Specifically, there have been known a so-called intravenous injection method and a sequence method for measuring the sensitivity of the baroreflex function according to reflection sensitivity appearing as extension or reduction of a heartbeat interval (an RR interval) that occurs together with a rise or a drop of a blood pressure. There has also be proposed a method of measuring the baroreflex function on the basis of a relation between fluctuation in a blood pressure and fluctuation in a pulse wave propagation time involved in the fluctuation in the blood pressure (Patent Literature 1).
However, the aortic arch and the carotid baroreceptor serving as a base of the baroreflex function has a characteristic that the aortic arch and the carotid baroreceptor emit impulses according to stretch of a blood vessel wall. Therefore, as explained above, there is a problem in that the baroreflex sensitivity measured using the blood pressure depends on vascular stiffness and is not accurately measured. That is, there is a problem in that the baroreflex sensitivity calculated from the blood pressure is a result better than an actual value as the blood vessel is softer and is a result worse than the actual value as the blood vessel is harder.
In order to solve such problems, in recent years, there has also been proposed a method of measuring the baroreflex sensitivity on the basis of a correspondence relation of a heartbeat interval confirming a change in the blood vessel diameter by tracking and measuring a pulsation change of a blood vessel diameter in the carotid using an ultrasonic echo without using a blood pressure (Non Patent Literature 1: hereinafter referred to as “echo method”). According to this echo method, theoretically, it is possible to measure neurogenic baroreflex sensitivity that is not affected by a blood pressure and vascular stiffness.